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Somatic Cells in Milk

The average dairy herd without a mastitis control program has subclinical or nondetectable mastitis in 47 percent of the cows. Three percent of the cows will be clinical at any point in time. The economic losses from both subclinical and clinical mastitis are great. The National Mastitis Council in 1978 estimated the yearly cost per cow from clinical mastitis cases, culling and decreased production to be about $161. Of this amount only about $56 was attributed to the costs of anitbiotics, discarded milk, time in treating and milking separately, veterinary service and culling prematurely. The greatest amount, $105, resulted from reduced milk production and income caused by subclinical mastitis. Infection in just one quarter reduces a cow's milk production 10 to 15 percent! Subclinical mastitis is the great hidden enemy of the dairyman.

The strip cup and visual observations at milking will not detect subclinical mastitis. The normal defense mechanism of the cow responds to subclinical mastitis (bacterial infection) by migrating leukocytes, a type of somatic cell from the blood, into the infected area of the udder. Leucocytes are divided into two types: lymphocytes and polymorphonuclear neutrophils. Macrophages, another type of defense mechanism cell, also migrate to the area of infection. The function of all these cells is to engulf and destroy the invading bacteria.

The level of somatic cells (leucocytes, macrophage and epithelial cells from the secretory tissue) in milk can be measured; therefore, it can become a rough measure of the level of infection in a herd. This can be done on the bulk tank milk sample, the composite milk sample from each cow, or on individual quarter milk samples.

There are three commercial test methods used presently to measure the number of somatic cells in farm milk samples: the California Mastitis Test (CMT), the Wisconsin Mastitis Test (WMT) and Electronic Cell Counting.

The CMT Test:

This test is commonly used on a composite milk sample taken from each cow. In the DHI program the test is run and read by the testing supervisor. It is also used by dairymen and veterinarians on individual quarters as a "cow-side test" indicator of the infection level in a cow's udder.

To conduct the test, two to three cc of the milk are placed in a four chambered paddle. An equal amount of an alkaline reagent is added to the milk. The reagent interacts with somatic cells in the milk to form a gel. After the paddle is tipped back and forth four or five times, a visual judgment reading of the amount of gel formation is taken.The sample is scored N (negative),T (trace), 1, 2 or 3 depending upon the amount of gel. A score of N or T indicates 500,000 or fewer somatic cells per cc of milk. These are acceptable scores and would indicate very little loss of production due to infection. Scores with higher ratings indicate higher somatic cell numbers and thus greater infection levels and milk losses.

The CMT reading usually increases as a cow moves into her second, third and later lactations. This is because cows usually have a greater amount of udder infection as they become older.

The CMT report is an early warning system that can identify cows with subclinical infections. The somatic cell program and culture results indicate that most cows have fewer than 400,000 somatic cells per cc milk are uninfected.

If a cow's composite milk sample has a CMT reading of two or three, each quarter should be sampled individually to determine if treatment is advisable.

The SCC count

A more accurate somatic cell count can be obtained by direct microscopic count or by electronic cell counting. The direct microscopic count is too slow; therefore, most somatic cell counts are done electronically with the Coulter or the Fossomatic counters. The cell count is expressed as thousands of cells per milliliter, or on a 0-9 scale. The somatic cell count is very important in detecting subclinical mastitis before any clinical mastitis signs are evident.

The WMT Test:

The Wisconsin Mastitis Test is commonly used by milk receiving plants to measure the quality of the farm bulk tank milk sample picked up by the milk hauler. The principle of the WMT is the same as the CMT. Instead of a subjective rating, the amount of gel that forms is measured in millimeters (mm) that remains in a calibrated tube. The WMT test is conducted under more precise procedures and standard temperature conditions. A WMT reading of 8 mm suggests that there are probably very few cows in the herd with subclinical mastitis. If the WMT reading reaches 20 mm or more, most milk plants will run a direct microscopic count to obtain an accurate somatic cell count. If the somatic cell count is 1,500,000 or greater, the dairyman is notified and assistance is provided in correcting the problem. Milking procedures and equipment operation need to be throughly checked.

Do not try to equate the WMT (mm) and the related cell count to the other scoring systems. The bulk tank sample is a composite sample representing the herd, while the other samples are composite samples from individual cows and will have a wider range of cell counts.

The importance of the somatic cell count relates not only to production loss from subclinical mastitis but also to the regulations governing Grade A and Manufacturing milk. The regulation for Grade A milk reads as follows: "Whenever the somatic cell count exceeds 1.5 million, the dairy plant is expected to send written notice to the producer. When two of the last four counts exceed that level, the written notice must come from the regulatory agency. The last notice remains in effect as long as two of the last four tests remain above 1.5 million in count. At the same time an inspection is required, also by the regulatory agency, and at some time between the third and 14th day following this inspection, a sample of milk is taken for further analysis. If this sample proves excessively high in cell count (over 1.5 million), action will be taken to suspend a producer's certificate to sell milk." Interstate milk shippers are required to meet these somatic cell standards. Any milk manufacturing plant that sells products to the U.S.D.A. has to meet similar regulations imposed by the U.S.D.A.

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This page was last updated on November 16, 2002